This invention relates to a power tool for two-step tightening of screw joints by a first high speed step for tightening the screw joint to a predetermined torque snug level and a second low speed step for tightening the screw joint to a desired final torque level.
In particular, the invention concerns a screw joint tightening tool with the above operation characteristics and comprising a rotation motor, an output shaft, a mechanical power transmission coupling the motor to the output shaft, and power supply means connected to the motor and including signal producing means for delivering a signal reflective of the output torque of the tool, and means for comparing the torque reflective signal with predetermined limit values corresponding to the torque snug level and the final torque level, respectively, and for delivering power shut-off initiating signals as the torque reflective signal attains these limit values.
Accordingly, the power tool according to the invention is intended to tighten screw joints in two subsequent steps which are both interrupted in response to signals produced as the first step torque snug level and the second step final torque level, respectively, are reached.
Power tools for two-step tightening are previously well known, an example of which is shown and described in U.S. Pat. No. 3,965,778. Although in this example, motor stall is used as a torque snug level indication it is as common to use a torque sensing transducer or other torque sensing signal producing means to initiate interruption of the first tightening step.
A problem concerned with two-step tightening power tools is that when being used on very stiff screw joints, i.e. screw joints with a very steep torque growth in relation to the angle of rotation or time, the inertia of the rotating parts of the tool causes a torque overshoot which even exceeds the desired final torque level to be reached by the second tightening step. This is due to the high rotation speed during the first tightening step and the sudden, steep torque growth in the joint.
Even at tools where the drive motor is braked electrically as the torque snug level is reached in order to absorb the remaining kinetic energy of the rotating parts, there will still be a torque overshoot, because the control system and the motor drive are not fast enough reacting to be able to avoid inertia influence on the torque level attained by the first tightening step.
One solution to this problem might be to employ a torque and speed responsive release clutch in the power transmission of the tool, a clutch that is set to release and limit the power transmission at the torque snug level during the first high speed tightening step but not to release during the second low speed tightening step.
A tool comprising a clutch of this type is described in U.S. Pat. No 4,881,435.
This prior art tool concept, however, brings another problem, namely the addition of a mechanical means that is subject to mechanical wear, which has a negative influence on the torque accuracy and the service life of the tool. It also requires a signal producing means for initiating power shut-off at release of the clutch. Such a signal producing means is mechanically coupled to the clutch and makes the tool undesirably complex.
The main object of the invention is to create a power tool for two-step tightening of screw joints, which tool includes means for initiating shut-off in response to a torque related signal reaching predetermined limit values representing a torque snug level and a final torque level, respectively, and which comprises a safety means for preventing overtightening of very stiff joints.
Another object of the invention is to create a power tool for two-step tightening of screw joints in which both steps are controlled in response to a signal reaching preset limit values representing a torque snug level and a desired final torque level, respectively, and in which a mechanical override safety clutch is arranged to limit the output torque during the first tightening step to a safety level well below the final torque level in cases of very stiff joints only.
This object is achieved by the invention as it is defined in the claims.
A preferred embodiment of the invention is hereinbelow described in detail with reference to the accompanying drawing.